Fracturing is an important process during the formation of some oil and gas wells, referred to as unconventional wells, to stimulate the flow of oil or gas from a rock formation.
Typically a borehole is drilled into the rock formation and lined with a casing. The outside of the casing may be filled with cement. The main purpose of the casing is to prevent the borehole from collapsing under the significant hydrostatic loading due to the rock above. It is not uncommon for boreholes to be several kilometres deep and they can be vertical as well as having horizontal paths depending on the rock strata and the application they are being used for.
The borehole casing is typically much smaller than the bore hole (for a 0.23-0.25 metre diameter bore hole, the external diameter of the casing might be 0.15-0.18 metres). The annulus between the casing and the bore hole is filled with cement which is pumped in from a pipe that is lowered to the bottom of the well and thereby feeds cement into the annulus so that it flows up the side of the casing to the surface. The casing serves two crucial purposes: (i) given that a well might be 5-10 kilometres underground, the cementation layer acts as a ‘glue’ between the casing and the rock so that the weight of the casing is carried by the rock (if the load isn't transferred to the rock then essentially you would be left with a 10 km long pipe hung from the surface. Under such loading conditions the casing would more than likely fail); (ii) the cementation layer acts as a seal to isolate each individual perforation track and to prevent any oil or gas from passing through the annulus and out of the well. It is noted that the Gulf of Mexico disaster was a result of the cementation layer failing (referred to as a well blow out). In that situation, the fluid is flowing out through the annulus and because it isn't flowing up through the casing, there will be no valves or control of any sort possible.
In unconventional wells the rock formation may require fracturing in order to stimulate the flow. Typically this is achieved by a two-stage process of perforation followed by hydraulic fracturing. Perforation involves firing a series of perforation charges, i.e. shaped charges, from within the casing that create perforations through the casing and cement that extend into the rock formation.
Once perforation is complete the rock is fractured by pumping a customised fluid, which is usually water based containing a variety of chemicals (often strong acids), down the well under high pressure. This fluid is therefore forced into the perforations and, when sufficient pressure is reached, causes fracturing of the rock.
A solid particulate, such as sand, is typically added to the fluid to lodge in the fissures that are formed and keep them open. Such a solid particulate is referred to as proppant.
The well may be perforated in a series of sections. Thus when a section of well has been perforated it may be blocked off by a blanking plug whilst the next section of well is perforated and fractured.
An example of a known perforator design is shown in FIG. 1. The perforator 10 comprises a generally cylindrical charge case 20 within which is mounted a shaped charge liner 30. The charge case is retained by an initiator holder 40 at a first end and is open at a second end 50.
The liner is generally conical in shape such that a volume is defined between the charge case and the liner which is filled with an explosive composition 60. In the oil and gas industry this composition typically comprises a variety of HMX based compositions in pressed powder form.
The liner 30 is placed within a charge case, which is filled with the main explosive. An initiator system is placed at the first end of the charge case, the initiator system being contained within the initiator holder. At the second end 50 of the charge case the base of the liner is open and is oriented in a radially outward direction when in use, facing the casing. In operation, the initiator system is operable to detonate the explosive composition which causes the liner material to collapse and be ejected from the charge case in the form of a high velocity jet of material. The jet breaches the wall of the perforator gun (see below) and the well casing, and then penetrates into the cementation layer and the rock, thereby causing a hole (a perforation tunnel) to form. The perforation tunnel provides the path between the well bore and the rock for fluid flow (i.e. either for hydraulic fracking or for oil/gas extraction).
It is noted that the liner shape can be chosen to suit the rock strata and application. Liners can be conical or hemispherical in general, conical liners typically giving more penetration than hemispherical liners, although there are variants on these shapes (e.g. tapered liners). The casing of the perforator is conventionally steel although other materials (such as brass and polymers) can be used depending on the particular application.
The shape of charge liners has been explored to some extent in the military and civil fields. For example, GB 1465259 discloses an explosive charge formed with a recess which is lined with a metal casing consisting of a plurality of triangular walls, wherein the mouth of the recess takes the shape of a plane polygon. The charge generates a very large number of high velocity splinters propelled in a given solid angle, and the thrust of the embodiments appears to be towards splinter dispersion rather than shaped charge effects. US 2011/0232519 discloses a shaped charge for use as a cutting tool which may have a polygonal shape. However, the liner has a recess in the form of a groove encircling an axis of symmetry so as to provide a cut pattern which is a polygonal pyramid, and is quite different to directional charges for fracking purposes.
Perforators may be arranged into a perforator gun which comprises a detonation cord which has perforator charges mounted thereon. The particular configuration within the gun is again dependent on the application. This can range from a helical arrangement with many thousands of charges along the gun at 13-20 spacing per metre over many tens of metres or hundreds of metres to other configurations where there is a sparse distribution of charges over 50 metres or so.
An example of a perforator gun is shown in FIG. 2 which shows a borehole 70 projecting through a rock formation 80. The rock comprises a number of bedding planes 90. Within the borehole is a metal casing 100 and the volume between the casing and the borehole has been filled with cement 110. A perforator gun 120 comprising a detonator cord 130 (and associated control circuitry) and a plurality of perforators 140 is located within the body of the perforating gun. Once detonated a perforator will eject a jet of material to form a hole (a ‘perforation tunnel’ located, for example, at 150) through the wall of the perforating gun, the well casing and the cementation layer into the rock formation.
The fracturing process is a key step in unconventional well formation and it is the fracturing process that effectively determines the efficiency of the well. The pressure, the amount of fluid and proppant and the flow rate are generally measured to help manage the fracturing process, including the identification of any potential problems (e.g. seal/plug failures). The down-hole temperature is likely to be in the region of 80-120° C., but can be as high as 170° C.
Rock formations that contain oil and gas deposits generally comprise rock strata that have aligned to form a number of bedding planes. Examples of such rock formations include oil/gas bearing shales in, for example, Canada, Dakota etc and oil/gas bearing tight rock formations in, for example, the North Sea.
Detonation of a perforator within the oil well will generally result in fractures appearing within the rock formation. The bedding planes represent a plane of least resistance for the growth of such fractures which may typically extend out from the bore hole by 50 metres.
If oil and gas deposits are situated such that they intersect a bedding plane then detonation of a standard perforator will enable the oil/gas to be extracted. However, in some instances the oil/gas deposits may be situated between bedding planes. In order to access these such deposits it would be preferable to have more control over the direction that fractures propagate in and, in particular, to be able to generate “out of bedding plane” fractures by means of the perforator gun.
It is noted that there are three general categories of well bore orientation: Where the well bore is orthogonal to the bedding planes (called a ‘vertical well’) Where the well bore is parallel to the bedding planes (called a ‘horizontal well’) Where the well bore runs at an angle across the bedding planes (called a ‘slant well’) (Note that the vertical and horizontal designations above relate to the bedding planes NOT the true geospatial coordinates.)
Known methods of encouraging out of plane fracture propagation include: increasing the pressure of the fluid that is pumped into the hole and including chemicals in the fluid that etch the rock in an effort to produce out of plane cracking. These techniques work well for some rocks and bedding plane configurations, but can be problematic for certain other environments (e.g. such as those in some tight gas wells).
It is therefore an object of the present invention to provide a shaped charge arrangement that facilitates preferential crack formation, growth and orientation in the rock strata.